In the current multi-carrier radio transmission systems, e.g., a mobile WiMAX evolution system, its pilot pattern design is illustrated in FIG. 1. In FIG. 1, the abscissa is a time axis with each grid representing one OFDM symbol, and the ordinate is a frequency axis with each grid representing one physical sub-carrier. Three radio time-frequency transmission blocks with the same size are shown respectively in FIGS. 1(a), 1(b) and 1(c). Each radio time-frequency transmission block consists of 6 consecutive OFDM symbols and 18 consecutive physical sub-carriers. In such a multi-carrier system, a minimum unit of radio transmission resources is called a minimum time-frequency cell grid, which consists of one OFDM symbol and one physical sub-carrier. In each time-frequency transmission block of a transmission link of the multi-carrier system, a pilot signal is transmitted in several minimum time-frequency cell grids. With the leftmost time-frequency transmission block in FIG. 1 as an example, a certain antenna in a cell transmits the pilot signal via the minimum time-frequency cell grid labeled as “1” In a time-frequency transmission block shown in FIG. 1, this antenna transmits a total of 6 pilot signals, since 6 minimum time-frequency cell grids in total in the time-frequency transmission block are labeled as “1”.
If there are multiple antennae in a cell, the minimum time-frequency cell grids occupied by the respective antennae usually are not overlapped in order to avoid interference. For instance, there are two antennae, i.e., the antenna 1 and the antenna 2, in a cell. As shown in FIG. 1, the pilot signal for the antenna 1, also called the pilot stream 1, is transmitted via the minimum time-frequency cell grid labeled as “1”. The pilot signal for the antenna 2, also called the pilot stream 2, is transmitted via the minimum time-frequency cell grid labeled as “2”. The pilot stream 1 and the pilot stream 2 are not overlapped either in a time domain or in a frequency domain. For a cell with more than two antennae, a similar method is employed to transmit its pilot streams.
If the multi-carrier radio transmission system is a cellular system, i.e., when a plurality of adjacent or overlapped cells need to transmit pilot signals, the minimum time-frequency cell grids occupied by these cells can be either overlapped or non-overlapped (i.e., no overlap between the respective pilot streams) in order to reduce interference. For instance, as shown in FIG. 1, each cell uses one pilot stream or two, which are called the pilot stream 1 and the pilot stream 2, respectively. Thus, the two antennae in the cell 1 use the 6 minimum time-frequency cell grids labeled as “1” and the 6 minimum time-frequency cell grids labeled as “2”, respectively. Similarly, the two antennae in the cell 2 use the 6 minimum time-frequency cell grids labeled as “3” and the 6 minimum time-frequency cell grids labeled as “4” respectively. Also, the two antennae in the cell 3 use the 6 minimum time-frequency cell grids labeled as “5” and the 6 minimum time-frequency cell grids labeled as “6”, respectively. Therefore, the 6 pilot streams altogether in the 3 cells of FIG. 1 do not overlap with each other. For the cell with more than two antennae, a similar method is employed to transmit its pilot streams. For the cell with one antenna, it will use only the pilot stream 1 or the pilot stream 2.
In the existing methods for transmitting pilot signals, no desirable transmission solution is available for a cell with more antennae. As shown in FIG. 1, if a cell uses not more than 6 antennae, the solution illustrated in FIG. 1 can be employed for transmission. However, if a cell uses more than 6 antennae, a position of the minimum time-frequency transmission block for transmitting pilot signals can not be determined by such a design. In addition, the existing methods for transmitting pilot signals are directed to the radio time-frequency transmission block composed of 6 consecutive OFDM symbols and 18 consecutive physical sub-carriers. If more than 6 antennae are used, one of problems to be solved is how to guarantee balance among the pilot streams for the respective antennae because the radio time-frequency transmission block is composed of 6 consecutive OFDM symbols. Particularly, if more than 6 antennae are used in a cell, tradeoff between the number of pilot signals and the system throughput should further be taken into account.